GO 326/ES 767 Volcanism of the Cascade Mountains

James S. Aber
Emporia State University

The Cascade Mountains consist of several active volcanoes (triangles) as well
as many volcanic centers that were active during the Neogene and Quaternary. These
volcanoes are developed above a subduction zone that stretches from northern California to
southern British Columbia. The subduction zone involves the Juan de Fuca and related oceanic
plates that descend beneath the western edge of North America.

Unlike typical subduction zones, no trench is present along the continental margin. Instead,
terranes and the accretionary wedge have been uplifted to form a series of coast ranges and
exotic mountains--Klamath Mountains. Inland from these coastal mountains are a series of
valleys--Shasta, Willamette, Puget. These valleys represent old forearc basins, which have been uplifted. Major cities of the region are located in these valleys today--Portland, Seattle and Vancouver.

Map modified from L. Freeman.

The Cascade Mountains include more than a dozen, large volcanoes. Although they share general
characteristics, each has unique geological traits and history. The major volcanoes of the
Cascades are listed below.

Major Cascade volcanoes. Listed from north to south;
with dates of historical eruptions. Data from Harris (1988).

Volcano

Location

Eruptions

Mt. Garibaldi

British Columbia

none

Mt. Baker

Washington

1792, 1843-65, 1870, 1880

Glacier Peak

Washington

1750 (?)

Mt. Rainier

Washington

1841, 1843, 1854

Mt. Adams

Washington

none

Mt. St. Helens

Washington

1800-57, 1980-84

Volcano

Location

Eruptions

Mt. Hood

Oregon

1854, 1859, 1865-66

Mt. Jefferson

Oregon

none

Three Sisters

Oregon

1853 (?)

Newbury Caldera

Oregon

none

Mt. Thielsen

Oregon

none

Crater Lake

Oregon

none

Mt. McLoughlin

Oregon

none

Volcano

Location

Eruptions

Medicine Lake

California

1910

Mt. Shasta

California

1786, 1855

Cinder Cone

California

1850-51

Lassen Peak

California

1914-17

Chaos Crags

California

1854-57

From this historical record emerge two geographic regions of current volcanic activity. Volcanoes are most
active in Washington and northern Oregon, and a second region of eruptions is northern California.
In contrast, central and southern Oregon are quiescent, as is southern British Columbia. The
sectors lacking modern eruptions correspond to positions of fracture zones that offset the Gorda,
Juan de Fuca, and Explorer oceanic ridges--Blanco, Nootka, and Sovanco fracture zones (see map above).

Cascade eruptions

Magma of the Cascades is generated by partial melting along the subduction zone. The most
typical lava of Cascade eruptions is andesite, an intermediate igneous composition which is characteristic for subduction
zones around the world. Other lava types span the spectrum of composition from basalt, to
dacite and rhyolite. Each major Cascade volcano has a distinct signature in terms of its
lava composition; some are quite consistent, whereas others show great variability.

Eruptions of andesite and basalt tend to be relatively calm events dominated by lava flows. Exceptions
occur where such magmas encounter substantial groundwater (under snow cover, glaciers), which may
create steam-driven explosions of the Strombolian type. Dacite and rhyolite magmas are
more viscous and tend to develop higher gas pressures. Such magmas may explode violently in
catastrophic eruptions that release immense volumes of tephra, ash, and pumice. Glacier
Peak, Mazama (Crater Lake), and Mt. St. Helens have experienced this type of eruption.

Recent eruptions, particularly Mt. St. Helens in 1980, have provided important clues for better
understanding of ancient volcanism in the Cascade system. One result of the 1980 eruption was
recognition of the importance of landslides and mass movements in the development of volcanic
landscapes. A huge section on the northern side of Mt. St. Helens slid away and created a jumbled
landslide terrain many miles from the volcano itself. Accompanying pyroclastic flows and
mudflows swept across the landscape. Similar events have occurred at Mt. Shasta and other
Cascade volcanoes in prehistoric times.

The eruption removed all or parts of most of the glaciers that had existed on the volcano
(Brugman and Post 1981). Sixty-two people died in the eruption and $1 billion was lost in the
lumber industry. The death toll would have been much higher on another day--the nearby timber
industry was shut down on Sunday when the eruption occurred.

Ancestral Mt. Shasta began to develop at least half a million years ago. The exact nature of
this volcano is unknown, but it must have been quite large, as it generated a tremendous
landslide between 300,000 and 360,000 years ago (Harris 1988). The enormous slide transported
chaotic blocks of rocks into Shasta valley. The avalanche deposits extend 45 km northwest of
the volcano and represent a volume of about 27 km³--about ten times the volume of the Mt.
St. Helens 1980 avalanche deposits.

Mount Shasta vicinity has experienced four main eruptive cycles during the past half million
years--Sargeants Ridge, Misery Hill, Shastina, and Hotlum. Hornblende and pyroxene andesite
lava flows dominate the eruptive cycles, which each terminate with intrusion of dacite domes
(Harris 1988). These domes, being more resistant to erosion, form the summit peaks of Mount
Shasta, Shastina, and Black Butte. The Hotlum eruptive center is considered to be active,
as the summit dome is still hot. Boiling sulfur springs and steam emissions were seen in
the 1850s, but these are reduced now to a single hot spring and small fumaroles.